Figure 4: a) Illustration of the nutshell structure
and movement of water droplet inside. b) Comparison of water transfer
time from outer shell surface to inner shell surface of four NSs
(thickness 1.6 mm); c) Evaluation of Water absorption and desorption
kinetics of AS, FS, PS, WS at varying time intervals. d) Sketch of water
transportation along cross-sectional and surface regions of WS,
emphasizing higher mobility in surface-driven transport. g) Illustration
of outer WS with a dense structure and inner WS with enhanced porosity,
making convergent-divergent channels.
To evaluate the ability of the NSs to absorb and release water, each of
the NS was immersed in DI water independently for a couple of days, and
the resulting weight was periodically measured. Subsequently, the wet NS
samples were subject to the ambient atmosphere (T= 25 ºC and 30%
Relative humidity) to evaluate the desorption of water because of the
spontaneous evaporation. Multiple samples (n=8) were examined to
validate the result. Figure 4 (c) demonstrates the
capacity to absorb and release DI water over different durations. As
anticipated, AS has the greatest capacity for absorption because of its
exceptionally porous structure and lowest desorption rate due to its
abundant vascular bundle,[45] causing it to retain
water for a longer duration. In contrast, WS desorbs the water quicker
than other NS. This might be attributed to the higher concentration of
hydrophobic lignin contents in WS, as presented in Table-S
1 . The lignin and cellulose contain hydrophilic hydroxyl (-OH),
carboxyl (-COOH) and carbonyl (C=O) functional groups and the aromatic
hydrophobic functional groups.[46] It has been
proven that the balanced combination of hydrophilic and hydrophobic
framework leads to enhanced evaporation
rate.[47,48] The cross-section of the NSs was also
experimented with for the water flow. Except for AS, all other NS
exhibits reduced water mobility along the dense cross-section compared
to the surface, as schematically illustrated in Figure 4 (d). The outer shell surface seems denser than the inner shell
surface, as schematically illustrated in Figure 4 (e). Therefore, the time duration of water transport differs between the
exterior and interior surfaces.
When each of the nutshells (NSs) is immersed in a small 1 ml droplet of
deionized (DI) water, the bottom surface becomes saturated, inducing
capillary flow to initiate and gradually spread the DI water to the top
surface. This process can be accelerated by placing another tiny 0.3 ml
DI water droplet on its upper surface. Capillary actions drive the
bottom water to transfer upwards, while gravity helps the top water flow
downwards. The phenomenon enabled by capillary forces and the directed
gravitational flow of water via micro/nanochannels is attributed to the
flow of water without using pumps. Subsequently, the whole porous
structure transforms from a dry to a completely wet state, and water
starts to evaporate naturally from the wet NS surfaces, as illustrated
in Figure 5 (a) . Under identical experimental conditions, open
circuit voltage (Voc) and short circuit current density
(Jsc) are recorded after the NSs are stabilized for 40
minutes at ambient conditions with 25% relative humidity (RH) and 25°C,
for the G-NS-G structure containing AS, FS, PS, WS as illustrated in